Packaging, whether rigid, semi-rigid, flexible, lidded, or collapsible, or a combination of these, serves not merely to contain the material being packaged but, depending on the nature of the material, to prevent ingress of harmful substances from the environment. Oxygen from the atmosphere has long been regarded as one of the most harmful substances for many packaged materials, especially foodstuffs.
Packaging made exclusively of glass or metal can provide an extremely good barrier both to egress of all substances from the package (especially water and carbon dioxide) and to ingress of all substances from the environment. Packaging made of polymers in whole or in part generally performs far less well in both these respects. This has restricted for many years the use of polymers in packaging, despite the great advantages of polymers. These advantages derive from the diversity of polymers themselves in mechanical, thermal, and optical properties and from the diversity and adaptability of fabrication techniques for polymers, allowing flexible bags, rigid containers, and clinging films to be made, the package wall being homogeneous, laminated, or coated. Compared with glass and metal packages, polymer packages are generally light and compared with glass are generally less breakable. There are also cost advantages with some polymers.
Polyethylene terephthalate is a major packaging polymer, used particularly for bottles for carbonated beverages. It is over twenty times less permeable than polypropylene while still having a practically significant permeability. There are extremely impermeable polymers such as copolymers of ethylene and vinyl alcohol, of vinylidene chloride and vinyl chloride, and of m-xylylenediamine and adipic acid ("MXD6"); but for practical or cost reasons these tend to be used as thin layers on or between polyethylene terephthalate or (in the case of MXD6) for blending with polyethylene terephthalate, in low percent quantities, still leaving practically significant permeability. For instance, oriented blends of polyethylene terephthalate (96%) and MXD6 (4%) are about 70% as permeable as polyethylene terephthalate. Chemical Abstracts, 1984, volume 100, abstract 100: 193165x, being an abstract of Japanese published patent application 58 160344, give some information on these blends.
We believe that there is considerable potential for extending the use of polymers by means of oxygen-scavenging systems. In these, oxygen reacts chemically as it is transmitted inwards towards the package contents. Accordingly, transmission of oxygen inwards to the package contents is reduced, not necessarily with any improvement in the performance of the package with respect to inward transmission of other substances such as nitrogen or water vapour or outward transmission of substances.
Among substances that we believe can then be more satisfactorily packaged with polymers we would particularly mention beers (especially lager beers), wines (especially white ones), fruit juices, some carbonated soft drinks, fruits, nuts, vegetables, meat products, baby foods, coffee, sauces, and dairy products. Almost all foods and beverages are likely to display some benefit.
Oxygen-scavenging implies consumption of a material incorporated in the wall of the package. This will be progressively consumed, so that the high barrier to oxygen must in principle be of limited duration. However, the deterioration of the barrier to oxygen is not necessarily commercially very significant. An advantage is obtained so long as the rate of such deterioration is not too great with respect to the time for which the deterioration can occur prior to consumption of the product. This will depend on the time from packaging to consumption and also on any relevant storage times of raw materials, fabricated packaging materials, and containers prior to their use in packaging the product. Good oxygen barrier performance over periods as short as one day might be in principle of use in certain cases, although periods of at least two, five, ten, twenty, fifty, or hundred days will extend the range of commercial applications. In respect of the prospective advantage from reducing barrier over short periods only, it should be remembered that oxygen entering the package shortly after the product is packaged has a longer time to react and therefore do damage than oxygen entering at a time nearer to consumption. It should also be remembered that in some cases oxygen will be packed with the product so that improvement of the performance of the package beyond a certain point may have a relatively insignificant effect on product quality.
An early proposal relating to oxygen-scavenging is described in U.S. Pat. No. 3,856,514 (published in 1971). This describes most particularly the addition of 0.8% to 2% by weight of antioxidants to hard polyvinyl chloride. Antioxidants exemplified are 2,2'-methylene-bis-(4-methyl-6-t-butylphenol) and 2,2'-dihydroxy-3,3'-dicyclohexyl-5,5'-dimethyldiphenylmethane. The best permeability value reported is twenty times lower than that of the polyvinyl chloride without the antioxidant. Experimental evidence on the duration of the effect is not given.
U.S. Pat. No. 4,048,361 (published in 1977) describes a multilayer structure in which a barrier layer such as an acrylonitrile-containing polymer, a terephthalate polyester, polyvinylidene chloride, a cellulosic material, or an elastomer is adhered to a layer comprising a carrier such as a polyolefin, polystyrene, and polyvinyl chloride and an antioxidant. No quantitative experimental investigation of the barrier properties is described. The use of antioxidants with polyethylene terephthalate is no specifically disclosed; in this respect it may be noted that antioxidants are not added to polyethylene terephthalate conventionally. (The conventional use of antioxidants is the suppression of oxidation of polymers, such oxidation in a package being regarded generally as undesirable.)
More recently, Rooney has described scavenging systems which operate by oxidation of organic materials such as 1,3-diphenylbenzofuran when illuminated in the presence of a dyestuff (Chem. Ind., 1979, 900-901; J. Food Science, 1981, 47, 291-298; Chem. Ind., 1982, 197-198). These systems have the disadvantage for use with, say, beer bottles that it is not practical to arrange for each bottle to be illuminated during storage.
As well as these proposals to use organic materials as scavengers there have been proposals to use inorganic reducing agents as follows: iron powder (Japanese published patent application 55 106519, published in 1980); hydrogen gas packed with the product (UK patent 1,188,170, published in 1970); and sulphites (UK patent specification 1,572,902, published 1980, and European published patent application 83 826 published 1983). There has been some commercial application of inorganic reducing agents. However, special packing procedures are of course necessary if hydrogen is used, and the use of sulphites and of iron requires special procedures for wall fabrication because of their poor compatibility with polymers.
Some discussion of the conventional measurements and units of oxygen permeation is appropriate at this point. The measurement is made by exposing a package wall of area A to a partial pressure p of oxygen on the one side and to an essentially zero partial pressure of oxygen on the other. The quantity of oxygen emerging on the latter side is measured and expressed as a volume rate dV/dt, the volume being converted to some standard condition of temperature and pressure. After a certain time of exposure (usually a few days) dV/dt is generally found to stabilise, and a P.sub.W value is calculated from the equation (1). EQU dV/dt=P.sub.W A p (1)
P.sub.W in the present specification and claims is called the permeance of the wall. (Analogy with magnetic permeance and electrical conductance would suggest that P.sub.W should be described as "permeance per unit area", but we are following the nomenclature in Encyclopaedia of Polymer Science and Technology, Vol. 2, Wiley Interscience, 1985, page 178.) The standard conditions for expressing dV/dt used generally and in this specification are 0.degree. C. and 1 atm (1 atm=101 325 N m.sup.-2). If the thickness of the area of wall is substantially constant over the area A with value T and the wall is uniform through the thickness (i.e. the wall is not a laminated or coated one) then the permeability of the material in the direction normal to the wall is calculated from the equation (2). EQU dV/dt=P.sub.M A p/T (2)
For non-scavenging materials, P.sub.W and P.sub.M are to a reasonable approximation independent of t and p, and P.sub.M of T although they are often appreciably dependent on other conditions of the measurement such as the humidity of the atmosphere on the oxygen-rich side and the temperature of the measurement.
For oxygen-scavenging walls, P.sub.W and P.sub.M are functions of t because the concentrations and activity of scavenger vary with time (particularly as the scavenger is consumed). This has not prevented us usually from measuring P.sub.W and P.sub.M reasonably accurately as a function of time (the changes in dV/dt being relatively gradual once the normal initial equilibration period of a few days is over). However, it should be recognised that, whereas after a few days' exposure to the measurement conditions a non-scavenging wall achieves a steady state in which dV/dt is equal to the rate of oxygen ingress to the wall, a scavenging wall achieves an (almost) steady state in which dV/dt is considerably less than the rate of oxygen ingress to the wall. This being the case, it is likely that P.sub.W calculated from (1) is a function of p as well as of t and that P.sub.M in (2) is a function of p and T as well as of t. P.sub.W and P.sub.M for scavenging walls are, strictly speaking, not true permeances and permeabilities at all (since permeation and scavenging are occurring simultaneously) but, rather, apparent ones. However, we have chosen to retain the conventional terms "permeance" and "permeability". So long as the conditions of the measurement are sufficiently specified they are suitable for characterising the walls in a manner relevant to the packaging user (i.e. in terms of the oxygen emerging from the wall).
All values of P.sub.W and P.sub.M hereinafter in this specification (except where stated otherwise) are to be understood to refer to conditions in which p=0.21 atm, the relative humidity on the oxygen-rich side of the wall is 50%, the temperature is 23.degree. C. and (in the case of P.sub.M values) the thickness of the wall of 0.3 mm. Conditions close to the first three of these, at least, are conventional in the packaging industry.
Further, as will be appreciated from the above discussion of the papers by Rooney, it is possible for P.sub.W and P.sub.M to be affected by the illumination of the wall under test. All P.sub.W and P.sub.M values hereinafter, and indeed all references to oxidation, oxidisability, and oxygen-scavenging properties, refer to the dark or else to conditions of irradiation not appreciably contributing to oxygen-scavenging.